The invention relates to a pulse generator for producing a voltage pulse for an essentially capacitive load, for use in an electrostatic dust extractor. Furthermore, the invention also relates to a method for producing a voltage pulse.
Electrostatic dust extractors (electrical filters) are used for retaining dust in fossil-fueled power stations and for numerous other industrial processes. For example, they are used in the paper, cement and metal-processing industries, in which dust extractors extract the dust contained in a gas flow charged by a strong electrical field. The dust is transported to an electrode, and is deposited there. Two problems, in particular, occur in this case, depending on the conductivity and distribution of the size of the dust particles:
i) Fine dust is often removed more poorly than coarse. Thus in the future, either complex downstream mechanical filters will be required, or existing filter systems will need to be retrofitted appropriately in order to comply with the increasingly more stringent legal requirements relating to dust emission limit values.
ii) Electrostatic charging of the already deposited dust layers frequently leads to flashovers, in which some of the deposited dust is released once again. In consequence, the efficiency of an electrical filter can be reduced considerably.
In order to avoid the abovementioned problems, it is known, for example from DE 196 14 195 C1, for electrostatic dust extractors to be used to which electrical high-power pulses are applied in order to increase the electrical field strength. This not only increases the amount of deposition, especially for fine dusts, but also reduces the charging of the deposited dust layers in the filter. In the past, comparatively long pulse durations of more than 50 xcexcs up to several milliseconds have been used, owing to the technical limits of production of the necessary electrical high-power pulses. These can be produced, with adequate life, using commercial power semiconductors, which are already available.
It is also known, from J. Miller, A. Schwab, xe2x80x9cFeinstaubabschaltung in betriebs-optimierten Elektrofilternxe2x80x9d [Fine-dust removal in operationally optimized electrical filters], Research Report FZKA-PEF 140, Karlsruhe University 1996, where the optimum pulse duration for use of this method is less than 10 xcexcs for voltage amplitudes from a few tens of kilovolts up to 100 kV, depending on the type of dust extractor. The pulsed currents to be switched in this case are several kiloamperes. However, no standard commercially available components yet exist by means of which pulses such as these can be produced with the necessary life of up to 10 billion pulses (corresponding to fault-free operation for a period of 1 year with a pulse repetition rate of 300 Hz).
DE 196 14 195 C1 therefore proposes the use of a combination of semiconductor components which have a long life but whose load is comparatively low, with subsequent pulse compression by means of a magnetic switch. For this purpose, the magnetic switch is connected in series with a thyristor switch, with at least two high-voltage pulse capacitors being used for energy storing and pulse forming. Major problems that have been found with this circuit concept are the high operating voltage of up to 60 kV for the thyristor switch, and the necessary heat extraction from, and possibly limited life, of the magnetic switch. Another problem is the use of resistors, which contribute considerably to power losses in the overall system since only a portion of the energy stored in the capacitive load can be recovered.
The electrostatic principles of pulse generators are described in the Journal of Electrostatics, 25 (1990), pages 23 to 40. In particular, this states that a tuned circuit is normally used and that either high voltage pulses are produced directly or else low voltages are converted to high voltages by means of a transformer. Furthermore, DE 41 04 386 A1 specifically describes a pulse generator for use in electric fence appliances, in which a pulse transformer is used.
Finally, GB 1 581 763 A discloses a pulsed voltage source for applying a voltage pulse to a capacitive load, which pulsed voltage source contains a DC voltage source and a chargeable capacitor which is connected to the DC voltage source. A pulsed voltage of predetermined magnitude, and with a predetermined pause, can be produced by an appropriate circuit. In this document, the DC voltage across the load is decoupled from the pulsed voltage source by using a second high-voltage appliance which is connected in series with the pulsed voltage source and through which the entire pulse current flows while the high-voltage pulse is being produced. The need to cope with the pulse current in the high-voltage appliance on the load side and the fact that the pulsed voltage source is at the high-voltage potential represent additional technical requirements and, technically, can be satisfied only with major complexity.
Against the background of the prior art, the invention is based on the object of specifying a pulse generator for producing a voltage pulse, which avoids the disadvantages mentioned above and improves the efficiency of existing systems without any major changes to them. Furthermore, the invention is based on the object of specifying a method for producing a voltage pulse.
According to the invention, the first-mentioned object is achieved by the features of the main apparatus claim 1, for example, and the second-mentioned object is achieved by the features of the main method claim 11, for example. Developments are specified in the respective dependent claims, for example.
In the invention, the pulse generator contains an energy-storage capacitor which is connected in series with a controllable switch. The switch is self-opening at the zero crossing of the current flowing in the load, in a series of tuned circuits. A rectifier, for carrying the current flowing in the opposite direction to the load after the zero crossing, is connected in parallel with the switch. If the load is essentially capacitive, the capacitance of the energy-storage capacitor is greater than the capacitance of the load and is at least ten times the capacitance of the load. Further, at least a first DC power supply is provided in order to charge the energy-storage capacitor.
The invention defines, in particular, such an arrangement in which
a pulse generator without a transformer is formed,
the amplitude of the output pulse is greater than the voltage of the energy-storage capacitor,
the recovery time of the switch is shorter than the period during which current flows in one current half-cycle,
a rectifier which is connected in parallel with the switch is provided to carry the current which flows in the opposite direction to the load after the zero crossing,
the energy-storage charge of the rectifier is small in comparison to the charge which is transferred to the load during the first current half-cycle, and
the current through the switching element and the rectifier is essentially equal to the current in the load.
In the method as claimed in patent claim 11, for example, an energy-storage capacitor which is connected in a series of tuned circuits is discharged to the load. This is done by closing and opening a controllable switch in a single discharging process during only one half oscillation period of the series tuned circuit. Further, in a second current half-cycle which is slightly shorter, it is charged once again from the energy temporarily stored in the load.
The measures as claimed in patent claims 1 and 11 for example, allow short-voltage pulses with an amplitude of several tens of kilovolts and pulse durations of a few microseconds to be produced with a high pulse repetition rate of several hundred Hertz. This can be done by using electrical components which are already commercially available without any problems, and which require neither a magnetic switch, whose life and power losses are limited, nor resistors, which also create losses.
Such a pulse generator according to the invention and such a method according to the invention can also be used for extracting sulfur oxides and nitrogen oxides from the gases emitted by the power stations. High-voltage pulses with an amplitude of several tens of kilovolts, a pulse duration of a few microseconds and pulse currents of several kiloamperes are also required for this purpose. Recent works have shown that, provided an appropriate power pulse technology is available, such methods operate economically, as is stated, for example, in G. Dinelli, xe2x80x9cAn integrated approach for flue gas cleaning by impulse energization techniquesxe2x80x9d, IEE Colloquium on xe2x80x9cCurrent environmental applications of electrostatic precipitationxe2x80x9d, digest no. 118, p. 6/1 et seq., London, UK 1991.
In addition, the pulse generator according to the invention and the method according to the invention can also be used for other applications, subject to appropriate adaptation of the passive components to the required pulse parameters. For example, they can be used for pulse production for the purification of engine exhaust gases using pulsed discharges or for plasma immersion ion implantation.
Since a controllable switch is provided which is self-opening at the zero crossing of the current flowing to the load, the control electronics required for controlling the switch are considerably simplified, since only the closing of the switch need be triggered.
In a further preferred refinement of the invention, a rectifier connected in parallel with the switch is provided in order to carry the current flowing in the opposite direction to the load after the zero crossing. In consequence, the voltage across the load is reduced rapidly, and the overall pulse duration is limited. Furthermore, if the load is essentially capacitive, at least some of the charge which is discharged to a capacitive load can flow back to the energy-storage capacitor once again.
Further advantageous refinements of the invention result from the respective dependent claims, for example.